Various types of speed reducers, in which a speed reducer is associated with an electric motor, have been conventionally known, for the purpose of obtaining high torque by use of a low rotational speed of the speed reducer. In most cases, components for a planetary gear-type speed reducer is assembled independently from assembling components for an electric motor, and the assembly of the speed reducer is then coupled together with the assembly of the electric motor to be a unit. For example, a speed reducer has been known, in which an input gear of a planetary gear-type speed reducer is associated, via pinion gears, with an output shaft of a motor. As another example, there is a speed reducer, in which an output shaft of a motor is fitted into a carrier arranged coaxially with an output shaft of a planetary gear-type speed reducer. As still another example, JP2000-120810A discloses a motor-incorporated speed reducer in which a rotor of a motor is integrated with a gear of a speed reducer (see paragraphs 0002-0007, paragraphs 0011-0019, and FIGS. 1, 3 and 4).
JP2000-118420A (see paragraphs 0011-0013, FIGS. 2 and 3) discloses a motor-driven power steering device having a hypocycloid gear mechanism having a single pinion that suffices for generating a high-speed reduction ratio, as illustrated in FIG. 2. More specifically, a motor shaft 32 is freely rotatably associated with an outer periphery of an output shaft 31. A rotor wound by coils 33 is arranged about the motor shaft 32. A permanent magnet 34 secured to a case is arranged at an outer periphery of the rotor. The motor shaft 32 extends along an axial direction of the output shaft 31. The motor shaft 32 is linked to the output shaft 31 via an offset shaft 35, a bearing 36, an eccentric gear 37 and a ring gear 38. The eccentric gear 37 is provided with a first gear portion 41, which rotates (revolves) in an eccentric manner along a ring gear 40 fixed to a case 39. A relationship between the first gear portion 41 of the eccentric gear 37 and the ring gear 40 establishes a hypocycloid gear mechanism.
In terms of reducing, via a speed reducer, a rotational speed outputted from a motor, i.e., in terms of increasing an output of a motor by use of a speed reducer, recent requirements have led to downsizing from the viewpoint of a limited space of an installation place. When a speed reducer is assembled independently from a motor, each of the speed reducer and the motor is required to have a shaft respectively. In such circumstances, a total axial length may be increased.
As aforementioned, when a rotor is integrated with a gear, a structure of a bearing may be complicated.
Further, for example, as is apparent from FIG. 2, a motor 1 is axially separated from the hypocycloid gear mechanism, which may increase an axial length.
The present invention has been made in view of the above circumstances, and provides a motor-incorporated hypocycloid-type speed reducer, in which a hypocycloid-type speed reducer is integrated with a motor, and an axial length of which is reduced.